[SECP256K1] Fix insecure links
Summary:
Backport of secp256k1#858
Update our specific files as well.
Test Plan:
Check it returns nothing:
git grep http://
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D9379